Processes to prepare high viscosity base oils are described in GB-A-1496045. In this publication a process is described wherein a vacuum residue of a crude petroleum source is first subjected to a propane de-asphalting step to obtain a de-asphalted oil (DAO). The DAO is further subjected to a furfural extraction process to remove polycyclic compounds, which are undesirable because of their low viscosity index and oxidative stability. The oil obtained after furfural extraction is subjected to a solvent dewaxing step in order to reduce the pour point of the oil. The dewaxed oil is finally hydrofinished.
U.S. Pat. No. 4,592,832 discloses a process to prepare a brightstock oil having a kinematic viscosity at 100° C. of 37 mm2/sec and a viscosity index of 95 as prepared from a light Arabian Vacuum Resid. The light Arabian Vacuum Resid is subjected to a propane de-asphalting step to prepare a DAO. The DAO is subjected to a N-methyl-pyrrolidone (NMP) solvent extraction step followed by dewaxing to obtain the Brightstock.
As a result of the above described process steps required to prepare a brightstock it is difficult to obtain a base oil having a combination of a high viscosity and a high viscosity index. Especially when starting from crudes having a lower content of saturates and/or VI contributing compounds than the light Arabian vacuum residue as illustrated in U.S. Pat. No. 4,592,832. This can be explained by the fact that the aromatic compounds as present in the DAO have a negative contribution to the viscosity index and have a relatively high viscosity whereas the saturates components have a lower viscosity contribution and variable VI contribution. Thus by removing these aromatic compounds in the solvent extraction step the viscosity index improves at the expense of the viscosity.
US-A-2003/0100453 discloses a blend of a Group I mineral derived base oil and Fischer Tropsch derived base oil. The blend is reported to show a better Oxidator A stability than the Fischer Tropsch oil, and a better Oxidator BN stability test than the Group I mineral derived base oil. A disadvantage of this blend is that at suitably high oxidative stabilities, solely those lube oils with suitably high viscosity index can be formulated that have a medium to low viscosity.